In a Halbach array magnet rotor, a predetermined number of main pole magnets and a predetermined number of yoke magnets are alternately arranged. Each of the main pole magnets is magnetized such that its radial inside portion and its radial outside portion have the opposite polarity from each other, of north pole and south pole. Also, each main pole magnet has its radial inside and radial outside polarities reversed from its closest main pole magnets. The yoke magnets are disposed to allow magnetic flux to flow through interfaces between the main pole magnets and the yoke magnets. The predetermined number is equal to the number of poles. In the Halbach array rotor, the magnetic flux is concentrated at the poles. Further, a yoke is not required. Therefore, this rotor can provide a compact, light weight, high performance rotary electric machine. As an example of this kind of rotary electric machine, it is known a light weight motor having a resinous rotor in which the main pole magnets and the yoke magnets are housed in a resinous hub, without requiring the yoke. This rotary electric machine is for example proposed in Unexamined Japanese Patent Publication No. JP-A-2000-197287.
However, the Halbach array rotor has following disadvantages.
First, because the main pole magnets and the yoke magnets are opposed to one another, the magnets cause demagnetization between them. As a result, they are likely to cause self-demagnetization. In a case of motor, it is difficult to supply a large amount of current. That is, load tolerance of the motor is low. Also, in a case of a generator, the rotor is affected by armature reaction. Therefore, if magnetic characteristic is reduced under a high temperature condition, the magnets are likely to be irreversibly demagnetized. Accordingly, the rotor is easily affected by heat. It is difficult to provide a high performance rotor with a compact body.
Next, because the main pole magnets and yoke magnets repel to one another with high forces, gaps are easily formed between the magnets. As a result, it is difficult to maintain rotation balance and to restore the balance in the rotor.
Further, the main pole magnets and the yoke magnets are arranged such that the same polarities are opposed at the radial outer portions, resulting the demagnetization. If the force of yoke magnets is too strong, the main pole magnets will be easily demagnetized. On the other hand, if the force of main pole magnets is too strong, the yoke magnets will be easily demagnetized. Especially in the generator in which the position of reaction field is not controlled, the armature reaction currents apply opposing magnetic fields to the magnets at many positions. Therefore, it is difficult to reduce the demagnetization.
Also, in the Halbach array rotor, the magnets having high magnetic flux density and providing a magnetic circuit are arranged throughout the outer periphery of the rotor. Accordingly, the costs is increased while the performance is improved.
Furthermore, because the main pole magnets in which the magnetic flux is concentrated is made of magnetic materials, it is difficult to provide locally high magnetic flux density as in core poles. Therefore, even in the Halbach array having the high concentration of magnetic flux, it is difficult to increase the total amount of magnetic flux linking to a stator.